Some of the world's most widely used climate emissions estimates could be missing far more pollution than anyone realized.

A new study from Northern Arizona University reports that the global greenhouse gas emissions database created by the Climate TRACE consortium, co-founded by former Vice President Al Gore, may be undercounting vehicle carbon dioxide emissions in cities by an average of 70%.

The findings, published in Environmental Research Letters, come as governments and cities increasingly rely on high-resolution emissions data to shape climate policy and track progress toward emissions goals.

Led by Kevin Gurney, a professor in NAU's School of Informatics, Computing, and Cyber Systems (SICCS), the study examined how Climate TRACE estimated emissions from cars and trucks and compared those figures against established transportation and fuel-use data. According to Gurney, the discrepancies -- combined with similar issues his team previously identified in power plant emissions estimates -- raise broader concerns about the reliability of rapidly emerging AI-driven climate monitoring systems.

"Given the importance of vehicle CO2 emissions in cities, we carefully examined the Climate TRACE data which relied on promising new artificial intelligence-based approaches," Gurney said. "When combined with our previous study on Climate TRACE power plant CO2 emissions, our results suggest that the Climate TRACE data significantly underestimate over half of U.S. fossil fuel-based CO2 emissions in cities."

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